Currently, wireless apparatuses are widely used mainly for business use in various scenes, such as, event preparation, security operations, and construction sites. In case of using wireless apparatuses for business use, approximately one to two hundreds wireless apparatuses are sometimes used in one scene. The use of wireless apparatuses in such a scene has a great advantage of allowing concurrent communication with a plurality of people. In addition, wireless apparatuses are characterized by ease of user operations for carrying out transmission and reception, resulting in prompt communication.
Wireless apparatuses communicate with each other by modulating a signal mainly according to a modulation system referred to as an analog modulation or a digital modulation, and then carrying out transmission and reception of an electric wave.
An analog modulation includes a Frequency Modulation (FM) system. By the FM system, the frequency of a carrier, an electric wave to be modulated, is changed to transmit the electric wave. Wireless communication by the FM system is used in, for example, FM radio broadcasting, amateur radio, and business radio.
A digital modulation includes a Frequency Shift Keying (FSK) system. By the FSK system, an electric wave is transmitted with the frequency of a carrier being set low when the digital data to be transmitted is zero, and being set high when the data is one. In this way, by the FSK system, two types of digital data are tailored to respective frequencies to transmit information. The FSK system includes a 4-FSK system which modulates four digital data, 00, 01, 10, and 11, into respective different frequencies. The 4-FSK system is widely used.
As described above, the modulation system used for communication depends on the type, the model, or the like of a wireless apparatus, and may not be the same. Generally, an old-modeled wireless apparatus often uses an analog modulation system, and a new-modeled wireless apparatus often uses a digital modulation system. In this case, the old-modeled and the new-modeled apparatuses cannot communicate with each other. For this reason, to a group having one hundred old-modeled wireless apparatuses, for example, ten new-modeled wireless apparatuses cannot be introduced. When a new-modeled wireless apparatus is introduced, all of the old-modeled wireless apparatuses are required to replace with new-modeled wireless apparatuses at once, which is not preferable in terms of cost.
To address the problem, Patent Document 1 discloses an invention of an apparatus which can reproduce a voice received even in an electric wave transmitted by either an analog modulation system or a digital modulation system. With this apparatus, even in a group having both new-modeled and old-modeled wireless apparatuses, a newly introduced new-modeled wireless apparatus can receive an electric wave transmitted by an old-modeled wireless apparatus and reproduce it.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-158725